A Parameterization of Heterogeneous Hydrolysis of N<sub>2</sub>O<sub>5</sub> for 3-D Atmospheric Modelling: Improvement of Particulate Nitrate Prediction

Abstract. Heterogeneous hydrolysis of N2O5 on the surface of deliquescent aerosol particles leads to HNO3 formation and acts as a major sink of NOx in the atmosphere during nighttime. The reaction constant of this heterogeneous hydrolysis is determined by temperature (T), relative humidity (RH), aerosol particle composition as well as the surface area concentration (S). However, its parameterization in previous 3-D modelling studies did not comprehensively consider these parameters. In this investigation, we propose a sophisticated parameterization of the heterogeneous hydrolysis of N2O5 with respect to T, RH, aerosol particle compositions and S, based on laboratory experiments. This new parameterization was incorporated into a 3-D fully online coupled model: COSMO-MUSCAT. As case study, we used the data from the HOPE-Melpitz campaign (10–25 September 2013). Here, we investigated the improvement of nitrate prediction over the western and central Europe. The modelled particulate nitrate mass concentrations ([NO3−]) were validated by filter measurements over Germany (Neuglobsow, Schmücke, Zingst, and Melpitz). The modelled [NO3−] were significantly overestimated for this period by a factor of 5–19, with the corrected NH3 emissions (reduced by 50 %) and the original parameterization of N2O5 heterogeneous hydrolysis. The proposed new parameterization significantly reduces the overestimation of [NO3−] by ~ 35 %. Particularly, the overestimation factor was reduced to approximately 1.4 within our case study period (September 12, 17–18 and 25, 2013), when [NO3−] was dominated by local chemical formations. Furthermore, the organic coating effect on a suppression of the N2O5 reaction probability may have been also significantly overestimated in previous modelling studies, due to a strong overestimation of the N2O5 reaction probability on coatings. Based on the original parameterization, previous studies reported a decrease of modelled [NO3−] up to 90 %, where both secondary organic aerosol (SOA) and N2O5 were built-up over western and central Europe. For this case study, the suppression of organic coating was negligible over western and central Europe, with influence on [NO3−] less than 2 % on average and 20 % at the most significant moment. As for a significant impact of the organic coating effect, N2O5, SOA and NH3 need to be present when RH is high and T is low. However, those conditions were rarely fulfilled simultaneously over western and central Europe. Hence, the organic coating effect on reaction probability of N2O5 over Europe may not be as important as expected in previous studies.